Influence of Blend Composition on Ultrafast Charge Generation and Recombination Dynamics in Low Band Gap Polymer-Based Organic Photovoltaics

A tremendous advancement in the performances of bulk heterojunction organic solar cells has been motivating a comprehensive investigation of many fundamental aspects regarding the structure-function relationship of these devices. The blend morphology has a crucial role in determining the device operation, affecting both charge separation and transport properties. Despite extensive spectroscopic investigations have been carried out to understand the fundamental photophysical processes, the charge separation mechanisms are still debated. Here we use ultrafast pump-probe spectroscopy to monitor directly photoexcited states dynamics in a promising polymer blend for photovoltaics, based on a low band gap polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), integrated with a fullerene derivative, [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We find that exciton quenching leads directly to two charge populations: charge-transfer states and free charges. Our experimental findings highlight how the different intrinsic charge generation pathways are strongly depending on the blend morphology and composition. The derived picture contributes to shed light on the fundamental photophysical processes that directly control the photovoltaic devices performances.